We examined the expression of the protein kinase C (PKC) δ subspecies in the trigeminal sensory system of the infrared-sensitive snake Trimeresurus flavoviridis. In the trigeminal ganglion (TG), diffuse low-intensity PKC δ immunoreactivity was found in TG neurons and fibers, while intense reactions were observed mainly in medium-sized neurons, which include most of the infrared-sensitive neurons. In the brainstem, intense PKC δ immunoreactivity was present in the intermediate layer of the optic tectum of the midbrain and in the nucleus descendens lateralis n. trigemini of the medulla oblongata; these areas are related to the infrared sensory pathway. In the pit organ (the infrared receptor), PKC δ immunoreactivity was present in terminal nerve masses in the pit membrane. These findings suggest that the PKC δ subspecies is involved in the infrared sensory pathway in the trigeminal sensory system of the infrared-sensitive snake.